There have been known acoustic wave devices including a comb-shaped electrode located on a piezoelectric substrate. Covering the comb-shaped electrode with a silicon oxide film to protect the acoustic wave device or compensate the temperature has been known as disclosed in, for example, Japanese Patent Application Publication Nos. 5-275963 and 2008-28980. Forming an insulating film, such as an aluminum oxide film, with a density different from that of the silicon oxide film on the silicon oxide film covering the comb-shaped electrode to achieve the reliability and the temperature characteristics has been known as disclosed in, for example, Japanese Patent Application Publication No. 2005-142629.
In a structure designed to have a comb-shaped electrode covered with an insulating film, an aperture for forming a pad is formed in the insulating film. In this structure, water may penetrate into the insulating film from the upper surface of the insulating film and the side surface of the insulating film in the aperture. The penetration of water into the insulating film deteriorates the characteristics.